There is increasing concern over the effects of new structures, and in particular large man made structures such as wind turbines, on the capability of new and existing radar systems, for example air traffic control, marine, and/or air defence systems.
Radar systems are generally designed to differentiate between radar returns containing reflections from many objects, both moving and stationary. Such reflected signals (collectively termed clutter) may for example originate from stationary objects such as trees, the ground and even the wind turbine towers themselves. Whilst existing radars may be designed to differentiate between clutter and moving objects based on the Doppler effect, there are many effects associated with structures such as wind turbines which contribute to a significant reduction in radar performance. The fact that large numbers of such structures are typically arranged in relatively close proximity to one another exacerbates the problem.
Wind farms, for example, typically comprise an array of large wind turbines, spaced out over an off-shore or inland area that may extend many kilometers. Each wind turbine typically comprises three principal elements: a tower, a nacelle and a blade assembly. The size and configuration of turbines may differ significantly from location to location (there are currently in the region of 40 or so different turbine designs in the UK alone). Generally, however, each turbine comprises a vertically-mounted blade assembly (having a horizontal rotational axis), and a tower exceeding a height of many tens of meters, or potentially over a hundred meters. The size of such structures, combined with the presence of large moving parts (e.g. the blade assemblies), means that the turbines act as effective scatterers of radio signals, with metal towers and/or blade assemblies in particular reflecting a high proportion of the transmitted signal back towards the radar and distorting returns from objects of interest. Thus, the turbines provide spurious moving targets for a radar system and cause shadowing or apparent modulation of signals associated with targets of interest, such as aircraft, marine vessels or the like.
Discriminating against spurious moving targets such as those associated with a moving blade assembly is complex and as such consumes significant additional processor time compared, for example, with simple static clutter reduction or the like.
The large size of the unwanted targets may cause undesirable effects such as saturation of a radar receiver, or the like. A large reflection, for example, can result in amplitude limiting within the receiver/signal processing thereby causing distortion and possibly resulting in reduced sensitivity and hence degraded detection capability.
Objects located behind the turbine(s) (from the perspective of the radar) may lie in the ‘shadow’ of the turbine. A large portion of the radar energy is blocked by the turbine and is thus lost by reflection in other directions. The radar energy that partially fills the shadow region behind the turbine (for example by diffraction) therefore represents only part of the original signal energy and so the field strength behind the turbine is diminished over a region behind the turbine. Shadowing may therefore result in missed detections.
The rotation of the blades also causes modulation effects, for example time modulation of the return signal as the blades present varying aspect angles, modulation or “chopping” of the radar cross section of objects behind the blade (as the blades intermittently obscure the returns from other objects), and Doppler modulation effects as a result of the blades' movement in the direction of the radar. Such modulation effects may cause a wanted target to be missed or to be mis-classified.
Other potential effects include the reflection and re-reflection (cascading reflection) of signals between turbines before they are returned to the radar.
Degradation in the capability of radar systems such as air traffic control systems to accurately detect and track targets of interest whilst discriminating against spurious targets is of particular concern because of the potential impact on aircraft safety.
There is therefore a need for improvements to enhance the function of important radar systems such as those used in air traffic control and air defence. There is also a more general need for radar systems which are resistant to the negative effects of large structures and in particular large man-made structures having moving parts, such as wind turbines.
The present invention aims to provide an improved radar system, useful in this and/or in other cluttered scenarios.
International Patent Application having publication number WO01/059473, which names Cambridge Consultants Ltd as patent applicant and whose disclosure is incorporated by reference, discloses a radar system which comprises apparatus for obtaining positional information relating to an object, the apparatus comprising: a warning zone definition stage for defining a warning zone (in two or three dimensions) within a detection field of the apparatus; and a discrimination stage for determining whether a detected object is within the warning zone; in which the warning zone is preferably defined as a three-dimensional region within the detection field.
International Patent Application having publication number WO97/14058, which names Cambridge Consultants Ltd as patent applicant and whose disclosure is incorporated by reference, discloses apparatus for and method of determining positional information for an object, including a method for determining the position of an object by means of detecting the relative timing of probe signals returned by said object at a plurality of spaced apart locations.